The fabrication of integrated circuits requires a method for accurately forming patterns on a semiconductor wafer. A photoengraving process known as photolithography, or simply masking, is widely employed for this purpose. The microelectronic circuit is built up layer by layer, each layer being based on a pattern received from a photolithographic mask. Such masks typically comprise a glass plate approximately the size of a wafer, the plate having a single pattern repeated many times over its surface. Each repeated pattern corresponds to a pattern to be imposed upon a layer of a wafer.
The mask patterns are derived from an optical reticle having a primary pattern which may be generated by a computer controlled light spot or electron beam which is scanned across a photosensitive plate. The reticle pattern is typically ten times the final size of the pattern to be imposed on the wafer. An image one-tenth the size of the reticle pattern is projected optically on the final mask. The reticle pattern is reproduced side by side many times on the mask, in a step-and-repeat process. Recent advances in reticle production have made it possible to produce reticles having patterns the same size as the final pattern. If such a reticle pattern could be aligned and focused onto a wafer, the mask fabrication could be substantially simplified or entirely eliminated thereby achieving a substantial savings.
The photolithographic process requires that each pattern on the mask be positioned accurately with respect to the layers already formed on the surface of the wafer. One technique is to hold the mask just off the surface of the wafer and to visually align the mask with the patterns in the wafer. After alignment is achieved, the mask is pressed into contact with the wafer. The mask is then flooded with ultraviolet radiation to expose photoresist on the surface of the wafer. The space between the wafer and the mask is often evacuated to achieve intimate contact; atmospheric pressure squeezes the wafer and the mask together. The latter apparatus is typically known as a contact printer. One defect of contact printers is that the masks quickly become abraded and useless. Since mask fabrication is expensive, it would be desirable to have another method that did not wear out the mask.
In view of the foregoing, a recent trend has been toward a technique known as projection alignment, in which an image of the mask pattern is projected onto the wafer through an optical system. In this case, mask life is virtually unlimited. However, one drawback has been that wafer sizes have been increasing, and the task of designing optics capable of projecting an accurate image over the larger area is becoming more difficult. Another drawback is the moveable projection optical system used in some machines for focusing a projected image onto a wafer. It is often difficult to focus such moveable optical systems and to hold the system in focus.
Recent projection aligners have attempted to circumvent the extreme difficulty of constructing a lens capable of resolving a micrometer-sized features over an area of many square inches. A much smaller area, on the order of one square centimeter, is exposed, and the exposure is repeated by stepping or scanning the projected image of the mask pattern over the wafer. Such machines are known as projection steppers. So far, all of the efforts to provide commercially acceptable projection steppers have been less than satisfactory. The use of one-to-one, or unit magnification, projection optics has become more frequent in such systems; however, achieving an adequately corrected, suitably large field in such systems has continued to present problems to those working in the art. As a result, a need has continued to exist for a projection stepping machine capable of using the now available, smaller reticles for directly forming patterns on wafers, thereby eliminating the need for a large, multiple pattern mask.
Unit magnification catadioptric lens system have been used in a variety of applications. U.S. Pat. No. 2,742,817 disclosed different types of unit magnification lenses, including one in which a concave spherical mirror was combined with a plano-convex lens, the lens being positioned between the mirror and the center of curvature of the mirror. The radius of curvature of the plano-convex lens was chosen so that it did not coincide with the center of curvature of the mirror, and the index of refraction and dispersion power of the lens were chosen so that the Petzval sum of the lens was numerically equal to the curvature of the mirror. The patent indicates that the single plano-convex lens can be replaced by a cemented doublet in order to achieve color correction but that in such a simple system, considerable difficulty is encountered in correcting various aberrations. The use of an achromatic doublet was said to permit the lens to be placed closer to the focal plane than would use of a single plano-convex lens.
An article entitled "Unit Magnification Optical System Without Seidel Aberrations" was published in July, 1959 in Volume 49, No. 7 of the Journal of the Optical Society of America at pages 713-716. The author, J. Dyson, disclosed that when the radius of a concave mirror and a plano-convex lens are chosen so that the upper principle focus of the lens lies on the surface of the mirror and so that the centers of curvature of the spherical surfaces coincide, then the flat face of the plano-convex lens will be a common object/image plane for the system. The sagittal image plane is flat; whereas, the tangential surface is a fourth order curve. The use of prisms to couple light into and out of the lens was disclosed, as was the use of a small air gap between the prism face and image/object surface. Such an air gap was said to introduce spherical and chromatic aberrations of a sense opposite to those provided by the lens so that by proper choice of the gap thickness and glass type, the author asserted, it would be possible to correct both types of aberrations simultaneously.
An article entitled "A Unit Power Telescope for Projection Copying" by C. G. Wynne appeared in 1969 in a book entitled Optical Instruments and Techniques at pages 429-434. A projection system was disclosed for use in making microcircuits by projection lithography. A concave spherical mirror, a meniscus lens, a plano-convex lens and a beam splitter were used. To correct the lens for both projection and alignment wavelengths, all spherical surfaces were required to be concentric; the material of the meniscus lens had to have a higher dispersion than that of the plano-convex lens; and to achieve chromatic correction of the Petzval curvature, the lenses had to have the same mean refractive index but different dispersions. The use of an air gap between the object/image planes and the surfaces of the beam splitter was disclosed. The author noted that "the spherical aberration caused by introducing a plane parallel air space is of opposite sign to that caused by introducing a plane parallel plate of higher refractive index. The Seidel spherical aberration of the plane parallel air space . . . can therefore be corrected by making the beam splitter cube of slightly higher refractive index than that of the plano-convex lens to which it is cemented; and by an appropriate choice of glass dispersion, chromatic aberration can be corrected at the same time. The higher orders of spherical aberration cannot be corrected, but for small air spaces, the uncorrected residuals are very small." The Wynne lens provided improved correction over a larger field; however, the lens was quite bulky, the air gap at the image and object planes was rather small and the orientation of the image and object planes made use of the lens rather impractical in a projection stepping system. A variation of the Wynne system is shown in U.S. Pat. No. 3,536,380 which disclosed a unit magnification lens in which the spherical surfaces are all concentric.
U.S. Pat. No. 4,103,989 disclosed a number of unit power concentric optical systems. The patentee departed from the formula suggested by Dyson for eliminating Seidel aberrations by changing the radius of the lens relative to the mirror; however, concentricity of the spherical surfaces was maintained. The use of prisms to separate the image and object planes is taught and such an arrangement is said to be superior to the beam splitter disclosed by Dyson.
U.S. Pat. No. 4,171,871 disclosed still another type of achromatic unit magnification optical system. In an effort to correct the lens over a very wide spectral range, the patentees permitted the spherical surfaces to be non-concentric and used as many as eight optical surfaces and six types of glass in an effort to correct the lens at all wave lengths within the range. A total thickness of glass of approximately 110 millimeters is used to achieve a rather small corrected field approximately 5 millimeters in height and an air gap of only 0.39 millimeters.